The present invention relates to the control of variable-pitch blades. It finds a particular application in the field of aeronautics, particularly for controlling the angular positions of guide vanes which guide the air inlet into compressors of turbomachines such as turbojet or turboprop aero engines.
Known devices for controlling variable-pitch blades in a turbomachine conventionally comprise a control member in the form of a ring surrounding a casing of the turbomachine and a number of levers, or rods, each rod having a first end connected to the control ring by an articulation and a second end mounted on a pivot of a respective blade.
Synchronized modification of the angular position of the blades is achieved by rotating the ring about the axis of the turbomachine. In order to be able to accompany the rotational movement of the ring, the connection between each rod and the ring has at least one degree of freedom in rotation about an axis directed approximately radially with respect to the ring. However, as the rod is mounted rigidly on the pivot of the corresponding blade, rotating the ring gives rise to other relative movements of the ring and that part of the rod that is mounted on the blade pivot. In order to tolerate these additional movements, or at least some of them, it is widely known practice for the connection to be produced in the form of a ball joint or similar component which, aside from rotating about an axis which is approximately radial with respect to the ring, allows rotation about an axis directed approximately circumferentially with respect to the ring. It is has also been proposed for a connection to be produced that offers an additional degree of freedom in translation in an approximately radial direction with respect to the ring. It is possible, among others, to refer to documents FR-A-2 608 678 or FR-A-2 746 141.
The use of mechanical articulations of the ball joint or similar type in a device for controlling a set of blades or, simultaneously, several sets of blades, entails the production of a great many parts at a relatively high cost. Furthermore, such articulations are subject to wear which may adversely affect their operation.
As an alternative, it has also been proposed for these movements, or at least some of them, to be tolerated through elastic deformation of the rod. Reference may be made, among others, to documents U.S. Pat. No. 4,367,264 and U.S. Pat. No. 4,979,874.
However, in those documents, the capacity for elastic deformation is provided by a thinning of the rod in its central part, between the ends connected to the control ring and to a blade pivot. Now, on the one hand, it is necessary to confer flexibility that is sufficient not to impede the transmission of movement between the control ring and the blades and, on the other hand, it is necessary to maintain a rod thickness that is great enough to withstand fatigue and avoid the risk of buckling. The best compromise between these contradictory requirements is difficult to find.
The object of the invention is to avoid the disadvantages of the devices of the prior art by proposing a control device using a flexible rod which has very good resistance to mechanical fatigue brought about by repeated actuation and very high resistance to buckling, while at the same time having a flexibility that allows it easily to deform in torsion, in the entire range of rotation of the control ring. Another object of the invention is to propose a device which allows its mounting to be simplified and also allows individual rods to be removed quickly and simply. Yet another object of the invention is to propose a device which has particularly low hysteresis at the blade/rod connection.
These objects are achieved by virtue of a device of the type comprising a rod, connecting means forming an articulation between a first end of the rod and a control ring, and means for fixing a second end of the rod to a pivot of a blade that is to be controlled, in which device, according to the invention, the said connecting means comprise a pivot passing through a first orifice pierced in the said first end of the rod and engaged in a radial housing of the said control ring, the said pivot being held in position by a locking ring which has openings for the passage of this pivot and which slides on the said control ring, the said fixing means comprise a fixing screw passing through a second orifice pierced in the said second end of the rod and introduced into a hole of the said pivot of the blade that is to be controlled, and the said device further comprises means for holding the said rod on the said blade pivot so that it can rotate without play.
With this particular structure, on the one hand, the device is far easier to mount/remove because of the monoblock nature (obtained by the presence of the locking ring) of the control ring and rods assembly and, on the other hand, the replacement of individual rods, without dismantling the control ring, is made particularly easy. Furthermore, automatic compensation for play in the blade-rod connection makes it possible to avoid, or at the very least to considerably reduce, any hysteresis.
According to a preferred embodiment, the said means for holding without play comprise a radial slot made in the said blade pivot and the width of which is narrower than that of the said second end of the rod that it is intended to accommodate. To do that, the said rod is advantageously curved about its longitudinal axis.
According to another embodiment of the invention, the said means for holding without play comprise at least one tenon made in the said blade pivot and intended to collaborate with tabs cut in the said second end of the rod. This cutout of the tabs preferably has a shape in accordance with FIG. 9.
According to one particularity of the invention, the said rod has, at its second end, a part which is wider than its first end and to which it gradually connects.
In order, in particular, to compensate for play between the rod and the control ring, the said radial housing is delimited by a liner fixed into the said first orifice of the rod. Likewise, in order to keep the rod pressed against the blade pivot, there is a stop inserted between the said second end of the rod and the said fixing screw.
Advantageously, the said rod is made of a laminated structure comprising thin rigid layers, formed of metal sheet, alternating with layers of elastomer, the layers being bonded together.
The presence of a laminated structure makes the rod behave in a remarkable way. Because of the thin nature of the metal layers, bonded together by layers of elastomer, the rod is easily deformable in bending and in torsion. In addition, there is no significant variation in the resistance to deformation throughout the range of operation of the control ring. The control of the pitch of the blades is therefore precise and easy. Furthermore, in spite of how thin they are, the metal layers together with the elastomer layers form an assembly which has very high resistance to fatigue and to buckling.
In one preferred embodiment that facilitates the mounting/removal of the rods, the said control and locking rings are each made in two parts, each extending over 180xc2x0 of the circumference of the turbomachine.